Transmitter circuit comprising a light source and a communication system

ABSTRACT

The invention concerns a transmitter circuit comprising a light source ( 20 ) and arranged to operate said light source ( 20 ) to transmit optical communication signals in response to balanced electric input signals from a first ( 11 ) and a second ( 12 ) circuit point. The transmitter circuit comprises a first ( 21 ) and a second ( 22 ) circuit branch which extend from said first ( 11 ) and second ( 12 ) points, respectively. The light source ( 20 ) is connected between the circuit branches ( 21, 22 ). The components which are positioned on the circuit branches ( 21, 22 ) are selected such that the transmitter circuit is formed with a symmetry which is such that, under normal operation conditions, a balanced drive voltage is the case between the connection points ( 13, 15 ) of the light source ( 20 ) on the circuit branches ( 21, 22 ) and such that the modulation current which drives the light source ( 20 ) essentially only depends on the voltage difference between said connection points ( 13, 15 ). The invention also concerns a communication system which comprises such a transmitter circuit.

BACKGROUND OF THE INVENTION AND PRIOR ART

The present invention concerns a transmitter circuit comprising a lightsource and arranged to operate said light source to transmit opticalcommunication signals in response to electric input signals from a firstand a second circuit point between which circuit points a balancedelectric input signal is intended to be present. The invention alsoconcerns a communication system comprising such a transmitter circuit.

Different transmitter circuits of the above mentioned kind are known.Such transmitter circuits are often used within telecommunicationsystems and data communication systems for transmitting optical signalsin response to electric signals. The optical signals are usuallytransmitted in one or more optical fibres. The electric input signal isusually present as a balanced signal on a pair of electric conductors,for example on a twisted pair. Such a pair of electric conductors has acertain characteristic impedance, for example 100 ohm. In order to avoidundesired reflections, such a pair of electric conductors should in itsend point be connected to a load which corresponds to the characteristicimpedance.

It should be noted that by a balanced signal is meant that the signalthat is present on the pair of conductors is such that the voltages oncorresponding points on the two conductors are of the same magnitude buthave opposite polarity relative to a reference potential. This referencepotential is usually earth potential. With an unbalanced signal (or“single-ended”) is meant that the signal, i.e. the voltage variation, isonly present on one conductor, while the other conductor, or thereference potential, is at a constant potential, usually on earthpotential.

On a pair of conductors with a balanced signal, due to noise or otherphenomena, a signal which is superposed on the two conductors may occur,a so-called common mode signal, which signal may vary with time. Thissignal is often undesired and should therefore be suppressed. This isoften done with the help of, for example, transformers, baluns (a balunis a device which converts a balanced signal to an unbalanced signal)and differential amplifiers.

Also when a balanced electric signal is to be converted to an opticalsignal, such an undesired superposed signal need to be suppressed inorder for the light source, which transmits the optical signal, to becorrectly operated. According to the prior art, this has usually beendone by first converting the balanced electric signal to an unbalancedelectric signal.

FIG. 1 shows an example of the prior art. The electric balanced inputsignal is here present on a twisted pair 30. The balanced signal isconverted to an unbalanced signal with the help of a balun 41 and atransformer 42. The circuit also comprises a termination resistance 43which is adapted to the characteristic impedance of the twisted pair 30.Thereafter follows one or more circuits 44, which i.a. produce asuitable bias current and a modulation current, wherein the totalcurrent drives the light source 20.

Also EP-A-0 542 480 shows an example of a transmitter circuit. Thetransmitter circuit comprises two differentiators and an amplifier fordriving a light emitting diode.

The prior known solutions are relative complicated and expensive, sincethey often comprise relatively complicated and expensive components,such as active components or transformers. Furthermore, knowntransmitter circuits often have a relatively high current consumption.

It should be noted that by active components is meant components whichproduce a gain or a switching, for example transistors, integratedcircuits, and diodes.

SUMMARY OF THE INVENTION

An object of the present invention is to achieve a transmitter circuitwhich is more simple than typical known transmitter circuits. A furtherobject is that the transmitter circuit should have a high functionalreliability and that it should be inexpensive to produce.

These objects are achieved according to the invention with a transmittercircuit of the kind which has been described in the first paragraphabove and which comprises a first circuit branch which extends from saidfirst point via a third point to at least a fourth point, and whereinsaid transmitter circuit comprises a second circuit branch which extendsfrom said second point via a fifth point to at least a sixth point,wherein said light source is connected between said third and fifthpoints, wherein the components which are positioned on said first andsecond circuit branches are chosen such that the transmitter circuit isformed with a symmetry which is such that under normal operationconditions a balanced drive voltage is the case between said third andfifth points, which balanced drive voltage only depends on the voltagedifference between said first and second points, wherein also themodulation current through the light source only depends on said voltagedifference.

With the invention, the advantage is achieved that the balanced signaldoes not need to be converted to an unbalanced signal. The circuitaccording to the invention can thereby be realised with simple andinexpensive components.

It should be noted that by “normal operation conditions” is meant thatthe transmitter circuit works within voltages and currents which arenormal for the transmitter circuit, where, as has been mentioned, alsoan undesired superposed voltage may be present on the balanced electricsignal. However, for example extreme voltage peaks may be considered toconstitute non-normal operation conditions.

According to a preferred embodiment of the invention, said first andsecond circuit branches are formed with a mirrored symmetry, such thatthe electric properties of the components which are arranged on saidfirst circuit branch correspond to the same electric properties of thecomponents which are arranged on said second circuit branch.

This mirrored symmetry may preferably be achieved if the first and thesecond circuit branches comprise components with exactly the same valueon corresponding positions in the respective circuit branch. The featurethat the electric properties of the components correspond to each othermeans however that it does not have to be exactly the components on thetwo circuit branches, as long as the electric properties of the twocircuit branches are the same. For example, the electric propertieswhich together are the case in the component or components which arearranged between two nodes in one of the circuit branches ought tocorrespond to the same electric properties which together are the casein the component or components which are arranged between thecorresponding two nodes in the second circuit branch.

Since the transmitter circuit is formed with this symmetry, it ispossible to, with simple components, maintain a balanced signal all theway to the light source. Furthermore, it is achieved that the lightsource is only modulated by the voltage difference between the abovementioned first and second circuit points. The current through the lightsource is thus independent of a possible common-mode signal which ispresent on said first and second circuit points.

According to a further embodiment of the invention, the transmittercircuit is arranged such that a first constant voltage is the case atsaid fourth point and a second constant voltage is the case at saidsixth point. Hereby, a suitable bias-current through the light sourcemay be obtained in a simple manner.

According to a further embodiment of the invention, the transmittercircuit is arranged such that at least one of said first and secondconstant voltages is adjustable. Hereby, the bias-current may simply beadjusted without influencing the modulation current.

According to another embodiment of the invention, the transmittercircuit comprises a first capacitor arranged on said first circuitbranch between said first and third points and a second capacitorarranged on said second circuit branch between said second and fifthpoints, wherein said first and second capacitors have essentially thesame value. Through these capacitors it is in a simple manner preventedthat a direct current, caused by the voltage difference between saidfourth point and said sixth point, is conducted past the light source.Hereby, the bias-current through the light source may be determined in asimple manner. Furthermore, energy is saved since no direct current isconducted past the light source.

According to a further embodiment of the invention, the transmittercircuit comprises a first resistance on said first circuit branchbetween said first and third points and a second resistance arranged onsaid second circuit branch between said second and fifth points, whereinsaid first and second resistances have essentially the same value. Withthe help of these resistances, the relationship between the voltage ofthe balanced input signal and the modulation current through the lightsource can be adjusted such that a suitable modulation of the lightsource is achieved.

According to still another embodiment of the invention, the transmittercircuit comprises a third resistance arranged on said first circuitbranch between said third and fourth points and a fourth resistancearranged on said second circuit branch between said fifth and sixthpoints, wherein said third and fourth resistances have essentially thesame value. With the help of these resistances, the voltage over thelight source is adjusted such that a suitable bias-current through thelight source is obtained.

According to a further embodiment of the invention, all components whichare arranged on said first and second circuit branches are passivecomponents. Hereby, the advantages of the invention are achieved in asimple manner and with inexpensive components. Preferably, notransformers or magnetic components are used in the transmitter circuit.As has been mentioned above, also no balun is used.

According to another embodiment of the invention, the transmittercircuit comprises a third circuit branch which extends between a pointon said first circuit branch and the corresponding point on said secondbranch, wherein the third circuit branch comprises at least a fifthresistance which is arranged to function as a termination resistance.The impedance of the circuit may thereby be adjusted to thecharacteristic impedance of a pair of conductors which pair conducts thebalanced electric signal to said first and second points.

According to a further embodiment of the invention, said third circuitbranch comprises a fifth and a sixth resistance which have essentiallythe same value and which are arranged to together function as atermination resistance, wherein the transmitter circuit is arranged suchthat a third constant voltage is the case between said fifth and sixthresistances on said third circuit branch. Hereby, a so-calledcommon-mode termination is achieved, which means that possible signalswhich are superposed on both the first and the second circuit points areterminated, which reduces the occurrence of reflections and otherdisturbing phenomena.

According to another embodiment of the invention, the transmittercircuit comprises transient protection connected to said first andsecond circuit branches and arranged to protect the light source againstundesired voltage pulses. The circuit is hereby protected against, forexample, electrostatic discharges.

According to a further embodiment of the invention, the transmittercircuit comprises a third capacitor arranged on said first circuitbranch between said first point and said first capacitor and a fourthcapacitor which has essentially the same value as the third capacitorand which is arranged on said second circuit branch between said secondpoint and said second capacitor, wherein said third circuit branchextends between a point on said first circuit branch positioned betweensaid first and third capacitors and a point on said second circuitbranch positioned between said second and fourth capacitors. With thehelp of these third and fourth capacitors, the input signal isAC-coupled. This, together with the above mentioned third constantvoltage and the fifth and sixth resistances, means that the input signalmay be adjusted to end up within an interval, in which the transientprotection works well.

As has been mentioned above, a further object of the invention is toachieve an advantageous communication system. This purpose is achievedwith a communication system which comprises at least a pair of electricconductors arranged to conduct a balanced electric signal, an opticalconductor arranged to conduct said signal in optical form, a transmittercircuit according to any of the preceding embodiments and a receiverunit, wherein said pair of electric conductors is connected to saidfirst and second points, said optical conductor is arranged to receiveand conduct on light from said light source and said receiver unit isarranged to receive said light from the optical conductor. Thecommunication system may, of course, also comprise another transmittercircuit and another receiver unit arranged to make a bi-directionalcommunication possible in the communication system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a transmitter circuit according to the prior art.

FIG. 2 shows a transmitter circuit and schematically a communicationsystem according to an embodiment of the invention.

FIG. 3 shows an equivalent circuit of the transmitter circuit accordingto FIG. 2.

FIG. 4 shows a transmitter circuit according to another embodiment ofthe invention.

FIG. 5 shows the principle of preferred embodiments of the invention.

DESCRIPTION OF EMBODIMENTS OF THE INVENTION

FIG. 2 shows a transmitter circuit and a communication system accordingto the invention. In the shown example, the communication systemcomprises a twisted pair 30 of conductors 31, 32. These conductors areconnected to a first 11 and a second 12 point of the transmittercircuit. It should be noted that other kinds of conductors than atwisted pair 30 are possible. For example, a ribbon table is thusconceivable or simply two conductors on a circuit card. A balancedelectric input signal is conducted to the first 11 and second 12 points.The transmitter circuit converts this signal to an optical signal whichis transmitted from a light source 20. The communication systemcomprises a receiver unit 37 and an optical conductor 35 which conductslight from the light source 20 to the receiver unit 37.

The transmitter circuit has a first circuit branch 21 and a secondcircuit branch 22. The first circuit branch 21 extends from the firstpoint 11 via a third point 13 to a fourth point 14. The second circuitbranch 22 extends from the second point 12 via a fifth point 15 to asixth point 16. The light source is connected between the third point 13and the fifth point 15. The first circuit branch 21 comprises a firstcapacitor C₁ and a first resistance R_(M1) which are connected in seriesafter each other between the first point 11 and the third point 13. In acorresponding manner, the second circuit branch 22 comprises a secondcapacitor C₂ and a second resistance R_(M2) which are connected inseries between this second point 12 and the fifth point 15.

Furthermore, the first circuit branch 21 comprises a third resistanceR_(B1) which is arranged between the third point 13 and fourth point 14.The fourth point 14 is arranged to be at a first constant voltage V_(A).In the shown example, this first voltage V_(A) is earth potential.Furthermore, the second branch 22 comprises a fourth resistance R_(B2)which is arranged between the fifth point 15 and the sixth point 16. Thetransmitter circuit is arranged such that a second constant voltageV_(B) is the case at the sixth point 16. One of said first V_(A) andsecond V_(B) constant voltages may suitably be adjustable. For example,the second constant voltage V_(B) may be adjustable. Thereby, thebias-voltage through the light source 20 may be simply adjusted withoutinfluencing the modulation current.

The transmitter circuit also comprises a third circuit branch 23. Thisthird circuit branch 23 extends from a point 17 on the first circuitbranch 21 to a point 18 on the second circuit branch 22. On the thirdcircuit branch 23, a termination resistance R_(T) is arranged. By asuitable choice of this termination resistance R_(T), the impedance ofthe circuit may be adapted to the characteristic impedance of theconduction pair 30 which is connected to the transmitter circuit. Thecomponents which are positioned on the first 21 and second 22 circuitbranches are chosen such that the transmitter circuit is formed with asymmetry. The symmetry is such that a balanced drive voltage is the casebetween the third 13 and the fifth 15 points. The balanced drive voltageis independent of a possible superposed voltage which is present on theinput signal, i.e. on the two first 11 and second 12 points. In thismanner, the light source 20 is modulated exactly in response to thevoltage difference between the two conductors 31, 32 which are connectedto the first 11 and second 12 points, respectively.

The easiest manner of achieving said symmetry is that the electricproperties of the components which are arranged between different nodeson the first circuit branch 21 correspond to the same electricproperties of the components which are arranged in correspondingpositions in the second circuit branch 22. This purpose may simply beachieved if the first capacitor C₁ has the same value as the secondcapacitor C₂, the first resistance R_(M1) has the same value as thesecond resistance R_(M2), and the third resistance R_(B1) has the samevalue as the fourth resistance R_(B2).

An advantage with the invention is that all components which arearranged on the respective circuit branch 21, 22 between the first 11and the fourth point 14 and between the second 12 and the sixth point16, respectively, may be passive components. In the shown case, thesecomponents consist only of capacitors and resistances. Hereby, also theuse of transformers or more expensive magnetic components is avoided.

A suitable bias-current through the light source 20 is selected by thechoice of the second constant voltage V_(B), the third resistanceR_(B1), and the fourth resistance R_(B2). The scaling factor between thevoltage of the balanced input signal and the modulation current throughthe light source 20 is selected by a suitable choice of the firstresistance R_(M1) and the second resistance R_(M2). The first C₁ and thesecond C₂ capacitors prevent a superposed voltage from reaching thelight source 20 in the form of a direct current.

In order to show that the current through the light source 20 isindependent of a possible superposed voltage on the conduction pair 31,32, reference is made to FIG. 3. FIG. 3 shows an equivalent circuit ofthe transmitter circuit according to FIG. 2. As a light source 20, forexample a light emitting diode or a laser diode may be used. A simplemodel of such a light source 20 is an independent voltage source V_(s)in series with a resistance R_(S). Z_(M1) corresponds to the firstcapacitor C₁ in series with the first resistance R_(M1). In acorresponding manner, Z_(M2) corresponds to the second capacitor C₂ inseries with the second resistance R_(M2). In FIG. 2 also the currentsI₁, I₂, and I_(S) as well as the voltages V₁, V₂, U₁, and U₂ are marked.

With reference to FIG. 3 the following equations may be formed.

$\begin{matrix}{I_{1} = \frac{V_{1} - U_{1}}{Z_{M\; 1}}} & (1) \\{U_{2} = {U_{1} + V_{S} + {I_{S} \cdot R_{S}}}} & (2) \\{I_{2} = \frac{V_{2} - U_{2}}{Z_{M\; 2}}} & (3) \\{I_{S} = {\frac{V_{B} - U_{2}}{R_{B\; 2}} + I_{2}}} & (4) \\{I_{S} = {\frac{U_{1}}{R_{B\; 1}} - I_{1}}} & (5)\end{matrix}$

Since the transmitter circuit is symmetrically formed, also thefollowing equalities are fulfilled.R _(B) =R _(B1) =R _(B2)  (6)Z _(M) =Z _(M1) =Z _(M2)  (7)

With the help of (1) to (7), the following expression may be derived.

$\begin{matrix}{I_{S} = \frac{{\left( {V_{2} - V_{1}} \right)R_{B}} - {V_{S}\left( {R_{B} + Z_{M}} \right)} + {V_{B}Z_{M}}}{{R_{B}\left( {{2Z_{M}} + R_{S}} \right)} + {Z_{M}R_{S}}}} & (8)\end{matrix}$

From (8) is clear that the current through the light source only dependson the difference between V₂ and V₁. If, for example, both V₂ and V₁suddenly increase, for example with 100 V, the current through the lightsource is not influenced.

In order to determine the bias-current, V₂ and V₁ may be set to be equal(V₂=V₁). Thereby, the following is derived.

$\begin{matrix}{I_{SB} = \frac{{V_{S}\left( {R_{B} + Z_{M}} \right)} + {V_{B}Z_{M}}}{{R_{B}\left( {{2Z_{M}} + R_{S}} \right)} + {Z_{M}R_{S}}}} & (9)\end{matrix}$

If it is assumed that Z_(M) is a resistance in series with a capacitor,as in FIG. 2, then Z_(M) goes towards infinity at the frequency 0 Hz.Thereby, the following is obtained when Z_(M) goes towards infinity.

$\begin{matrix}{I_{SB} = \frac{V_{B} - V_{S}}{{2R_{B}} + R_{S}}} & (10)\end{matrix}$

The expression (10) thus shows the direct current (the bias-current)through the light source. The modulation current is the total current(8) minus the bias-current (9). The modulation current is thus:

$\begin{matrix}{I_{SM} = \frac{\left( {V_{2} - V_{1}} \right)R_{B}}{{R_{B}\left( {{2Z_{M}} + R_{S}} \right)} + {Z_{M}R_{S}}}} & (11)\end{matrix}$

In order to take a numerical example, it may for example be assumed thatthe light source is a laser with V_(S)=1.6 V and R_(S)=30 ohm.Furthermore, it may for example be assumed that V_(B)=+5 V. If, forexample, a bias-current of 8 mA is desired, then the following isobtained with the help of (10).R_(B)=197.5 ohm

If it is assumed that the modulation current should be 1 mA at 1 Vdifference between V₁ and V₂, and if it is assumed that the capacitorscan be seen as short-circuited at the modulation frequency, then R_(M)is obtained to the following with the help of (11).R_(M)=450.8 ohm

It remains to determine R_(T) such that the total impedance matches thebalanced input impedance of the conductor pair. Without R_(T) it is thecase at higher frequencies (Z_(M)=R_(M)), that the input impedance isthe following.

$\begin{matrix}{R_{IN} = {{2R_{M}} + \frac{2R_{B}R_{S}}{{2R_{B}} + R_{S}}}} & (12)\end{matrix}$

If the obtained numerical values are inserted, then the following isobtained.R_(IN)=929.6 ohm

If, for example, a total input impedance of 100 ohm is desired, thenR_(T) gets the value 112.1 ohm.

From the above described example, it is clear that the invention worksas it is intended to work and that the circuit can be dimensioned in asimple manner.

FIG. 4 shows another embodiment of the invention. The transmittercircuit according to FIG. 4 differs from the transmitter circuitaccording to FIG. 2 in that the third circuit branch 23 comprises afifth resistance R_(T1) and a sixth resistance R_(T2). These resistanceshave essentially the same value. Furthermore, the third circuit branch23 is arranged with a third constant voltage V_(C) between said fifthR_(T1) and sixth R_(T2) resistances. Furthermore, the transmittercircuit comprises a transient protection 27 arranged to protect thelight source 20 against undesired voltage pulses. Furthermore, the firstcircuit branch 21 of the transmitter circuit comprises a third capacitorC₃. The second circuit branch 22 comprises a fourth capacitor C₄. Inorder to achieve a suitable symmetry, suitably the third capacitor C₃has the same value as the fourth capacitor C₄.

The transient protection 27 may be realised in different manners knownto the person skilled in the art. For example, diodes or zener diodesmay be used in order to limit the voltage if it ends up outside acertain interval. With the help of the third capacitor C₃ and the fourthcapacitor C₄, the signal has been AC-coupled before it reaches thetransient protection 27. With the help of the third constant voltageV_(C) and the fifth R_(T1) and sixth R_(T2) resistances, it is securedthat the input signal is around the third constant voltage V_(C) whichis adjusted to the transient protection 27. It is thereby achieved thatthe transient protection 27 only limits the voltage if non-normalvoltages occur. Through the third constant voltage V_(C) and the fifthR_(T1) and sixth R_(T2) resistances, also reflections and other problemsare reduced, since a so-called common-mode termination is achieved whichmeans that signals which are common to the two conductors areterminated.

FIG. 5 shows the principle of the invention. As is indicated withhatched lines in FIG. 5, the transmitter circuit may comprise furthercross-connections between the first circuit branch 21 and the secondcircuit branch 22. It is even possible that the transmitter circuitcomprises active components. However, preferably passive components areused. Concerning the components which are important for the normaloperation of the transmitter circuit, it is preferably the case thatthese components are arranged such that the transmitter circuit isformed mirror-symmetrical along a symmetry line 36 which passes throughthe middle of possible cross-connections. Thereby, the above describedadvantages of the invention are achieved in a simple manner. Certainparticular components, such as transient protection, which do not haveany influence on the normal operation, do not necessarily have to bearranged with the mirrored symmetry. It should also be noted that thetransmitter circuit may comprise further components. For example, thetransmitter circuit may be arranged with a low-pass filter forpreventing high frequency signals from reaching the light source.

The invention has several advantages, such as has already been describedabove. The input signal does thus not have to be converted into anunbalanced signal. This means i.a. that the voltages at the points 13and 15 will be in opposite phases, which means that disturbances whichcould reach other components will be small, since such disturbances fromthe points 13 and 15 tend to cancel each other.

A transmitter circuit according to the invention may suitably form partof a device which is arranged in a home or at a working place in orderto convert electric signals, for example from a computer, to opticalsignals which are transmitted via an optical fibre. Such a device mayadvantageously be arranged in or in connection to a wall in a room. Thetransmitter circuit according to the invention may also form part of acentrally positioned device which transmits optical signals to one ormore optical fibres in order to transfer these optical signals to a homeor to another place.

The invention is not limited to the shown embodiments but may be variedwithin the scope of the following claims.

1. A transmitter circuit comprising a light source (20) and arranged tooperate said light source (20) to transmit optical communication signalsin response to electric input signals from a first (11) and a second(12) circuit point between which circuit points a balanced electricinput signal is intended to be present, wherein said transmitter circuitcomprises a first circuit branch (21) which extends from said firstpoint (11) via a third point (13) to at least a fourth point (14) andwherein said transmitter circuit comprises a second circuit branch (22)which extends from said second point (12) via a fifth point (15) to atleast a sixth point (16), wherein said light source (20) is connectedbetween said third (13) and fifth (15) points, wherein the componentswhich are positioned on said first (21) and second (22) circuit branchesare chosen such that the transmitter circuit is formed with a symmetrywhich is such that under normal operation conditions a balanced drivevoltage is the case between said third (13) and fifth (15) points, whichbalanced drive voltage only depends on the voltage difference betweensaid first (11) and second (12) points, wherein also the modulationcurrent though the light source (2) only depends on said voltagedifference.
 2. A transmitter circuit according to claim 1, wherein saidfirst (21) and second (22) circuit branches are formed with a mirroredsymmetry, such that the electric properties of the components which arearranged on said first circuit branch (21) correspond to the sameelectric properties of the components which are arranged on said secondcircuit branch (22).
 3. A transmitter circuit according to claim 1,arranged such that a first constant voltage (V_(A)) is the case at saidfourth point (14) and a second constant voltage (V_(B)) is the case atsaid sixth point (16).
 4. A transmitter circuit according to claim 3,arranged such that at least one of said first (V_(A)) and second (V_(B))constant voltages is adjustable.
 5. A transmitter circuit according toclaim 1, comprising a first capacitor (C₁) arranged on said firstcircuit branch (21) between said first (11) and third (13) points and asecond capacitor (C₂) arranged on said second circuit branch (22)between said second (12) and fifth (15) points, wherein said first (C₁)and second (C₂) capacitors have essentially the same value.
 6. Atransmitter circuit according claim 1, comprising a first resistance(R_(M1)) arranged on said first circuit branch (21) between said first(11) and third (13) points and a second resistance (R_(M2)) arranged onsaid second circuit branch (22) between said second (12) and fifth (15)points, wherein said first (R_(M1)) and second (R_(M2)) resistances haveessentially the same value.
 7. A transmitter circuit according to claim6, comprising a third resistance (R_(B1)) arranged on said first circuitbranch (21) between said third (13) and fourth (14) points and a fourthresistance (R_(B2)) arranged on said second circuit branch (22) betweensaid fifth (15) and sixth (16) points, wherein said third (R_(B1)) andfourth (R_(B2)) resistances have essentially the same value.
 8. Atransmitter circuit accordint to claim 7, comprising a third circuitbranch (23) whict extends between a point (17) on said first circuitbranch (12) and the corresponding point (18) on said second circuitbranch (22), wherein the third circuit branch (23) comprises at least afifth resistance (R_(T)) which is arranged to function as a terminationresistance.
 9. A transmitter circuit accordint to claim 8, wherein saidthird circuit branch (23) comprises a fifth (R_(T1)) and sixth (R_(T2))resistance which have essentially the same value and which are arrangedto together function as a termination resistance, wherein thetransmitter circuit is arranged such that a third constant voltage(V_(C)) is the case between the fifth (R_(T1)) and sixth (R_(T2))resistances on said third circuit branch (23).
 10. A transmitter circuitaccording to any one of claims 5 and 9, comprising a third capacitor(C₃) arranged on said first circuit branch (21) between said first point(11) and said first capacitor (C₁) and a fourth capacitor (C₄) which hasessentially the same value as the third capacitor (C₃) and which isarranged on said second circuit branch (22) between said second point(12) and said second capacitor (C₂), wherein, said third circuit branch(23) extends between a point (17) on said first circuit branch (12)positioned between said first (C₁) and third (C₃) capacitors and a point(18) on said second circuit branch (22) positioned between said second(C₂) and fourth (C₄) capacitors.
 11. A transmitter circuit according toclaim 1, wherein all the components which are arranged on said first(21) and second (22) circuit branches are passive components.
 12. Atransmitter circuit according to claim 1, comprising transientprotection (27) connected to said first (21) and second (22) circuitbranches and arranged to protect the light source (20) against undesiredvoltage pulses.
 13. A communication system comprising at least a pair(30) of electric conductors (31, 32) arranged to conduct a balancedelectric signal, an optical conductor (35) arranged to conduct saidsignal in optical form, a transmitter circuit according to claim 1, anda receiver unit, wherein said pair of electric conductors (31, 32) areconnected to said first (11) and second (12) points, said opticalconductor (35) is arranged to receive and to conduct on light from saidlight source (20) and said receiver unit is arranged to receive saidlight from the optical conductor (35).